1. Field of the Invention
The present invention relates generally to a semiconductor process, and more specifically to a semiconductor process, which forms a silicon rich layer on the sidewalls of an isolation structure so that an epitaxial structure can grow on the silicon rich layer.
2. Description of the Prior Art
For decades, chip manufacturers have made metal-oxide-semiconductor (MOS) transistors faster by making them smaller. As the semiconductor processes advance to very deep sub micron era, such as 65-nm node or beyond, how to increase the driving current for MOS transistors has become a critical issue.
In order to improve device performances, strained-silicon technology has been developed. Strained-silicon technology is becoming more and more attractive as a means for getting better performances in the field of MOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strain makes MOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel.
However, some problems occur while applying strained-silicon technologies. By using a fin-shaped field-effect transistor as an example, the fin-shaped field-effect transistor includes at least a fin-shaped structure and isolation structures that are located between the fin-shaped structures. As epitaxial structures grow on the fin-shaped structures by applying strained-silicon technologies, gaps are generated between the epitaxial structures and the isolation structures because the epitaxial structures can not grow on the isolation structures. Since the epitaxial structures can not fill up spaces between the isolation structures, the volume of the epitaxial structures tend to be reduced and the desired shapes for the epitaxial structures can not be obtained. Thus, the performances of the epitaxial structures would decrease. Besides, structures such as a gate structure, an interdielectric layer, a contact hole etch stop layer, a metal plug or the likes formed above the epitaxial structures cannot contact the epitaxial structures smoothly, thereby decreasing the quality of the fin-shaped field-effect transistor.